Field
Embodiments of the present invention are generally directed to radar, and more particularly, to a methodology for automated cancellation of harmonics using feed forward filter reflection for radar transmitted linearization.
Description of Related Art
Harmonic radar exploits harmonically generated returns from nonlinear targets to aid in their detection. The advantage of nonlinear radar over traditional radar is its high clutter rejection, as most naturally-occurring (clutter) materials do not exhibit a nonlinear electromagnetic response under illumination by radio-frequency (RF) energy. The disadvantage of nonlinear radar is that the power-on-target required to generate a signal-to-noise ratio (SNR) comparable to linear radar is much higher than that of linear radar. Nevertheless, nonlinear radar is particularly suited to the detection of man-made electronic devices, typically those containing semiconductors whose radar cross-section is very low owing to their thin geometric profile.
A nonlinear radar tailored to a set of RF electronic responses would help law enforcement agents locate devices whose emissions exceed those permitted by law, allow security personnel to detect unauthorized radio electronics in restricted areas, or enable first-responders to pinpoint personal electronics during emergencies such as immediately after an avalanche or earthquake.
Harmonic radar is a type of nonlinear radar that transmits a single frequency f0 and receives one or more integral multiples of that same frequency (e.g. 2f0, 3f0, 4f0, etc.). The most common harmonic radars receive the lowest harmonic, 2f0, because 2f0 tends to be the strongest of all harmonics generated by an electronic target for a given transmit frequency and power.
In order to generate a detectable harmonic response from an electronic device, the required power density on dBmW target is approximately
      10    ⁢                  dBm        ⁢        W                    cm        2              ,which is comparable to the power density observed directly below a cellular base station. Thus, the harmonic radar's transmitter must provide high power to overcome interference by possible cellular towers in the vicinity. Also, typical harmonic responses are received power levels of −100 dBm, and this weak signal must not be masked by harmonics generated by the transmitter that are coupled directly to the receiver. Thus, the harmonic radar's high transmit power must be provided with high linearity.Basic Harmonic Radar
A simple harmonic radar is shown in FIG. 1A. The transmitter consists of a synthesizer which outputs a single frequency f0, a power amplifier (PA) which boosts the transmit signal to a level suitable for exciting a harmonic response from the target, and a transmit (Tx) antenna. The single frequency f0 illuminates the target and the harmonic 2f0 radiates from the target back towards the radar. The receiver consists of a receive (Rx) antenna, a low-noise amplifier for boosting the received signal to a level suitable for capture, and an analog-to-digital converter (ADC) which records the received signal. Target detection may be performed using this continuous-wave configuration. Ranging may be accomplished by pulsing or otherwise modulating the transmission.
Unfortunately, practical harmonic radar design is not so straightforward, as illustrated in FIG. 1B. Three reasons highlighted below are:
The power amplifier, in addition to boosting the transmitted tone, generates harmonics. If the amplifier-generated harmonic 2f0 is not attenuated sufficiently before it arrives at the Tx antenna, this harmonic will be radiated from the transmitter, reflect from the target, and mask the target's harmonic response.
In any practical radar system (linear or nonlinear), coupling exists between the transmitter and the receiver. If this coupling is excessively high, the transmitter-generated 2f0 will be fed directly to the receiver and will mask the target's harmonic response.
The target's linear response (at f0, reflected from the target casing) will likely be much stronger than its nonlinear response (at 2f0, radiated from the target electronics). Thus, even if the Tx/Rx antenna coupling is minimal, a strong signal at f0 will enter the receiver at the Rx antenna. If this received f0 is not attenuated sufficiently before the low noise amplifier (LNA), the LNA will pass f0 to the ADC (possibly saturating the converter) and/or it will produce its own 2f0 to mask the target's harmonic response.
For these highlighted reasons, reduction of the system-generated harmonics, i.e. “linearization” of the radar, is necessary.
Linearized Harmonic Radar
Two popular techniques for RF linearization are filtering and feed-forward cancellation. Filtering removes system-generated harmonics by attenuating or reflecting them at the output of the nonlinearity. Feed-forward cancellation adds a phase-shifted version of the undesired signal to the combined signal in order to remove the undesired signal. The undesired signal may be a harmonic, or it may be a strong linear signal that is likely to generate a harmonic.
Filtering may be implemented in the transmitter and/or the receiver. It is depicted in FIG. 2 as part of the transmitter identified as (A). Here, a lowpass filter removes 2f0 at the output of the amplifier, which prevents the amplifier-generated 2f0 from radiating out of the Txantenna. Cancellation is usually implemented in the receiver. It is depicted in FIG. 2 identified as (B) inserted at the junction between the Rx antenna and the LNA. A 180° phase-shifted version of f0 is added to the signal received from the Rx antenna, where the signal includes the target response at f0 as well as 2f0. The vector sum of the phase-shifted f0 with the un-shifted f0 is ideally zero, ensuring that only 2f0 appears at the output of the cancellation circuit and continues along the receiver chain.
The architecture presented in FIG. 2 is still not an adequate nonlinear target detector for practical standoff ranges between the Tx/Rx antennas and an electronic target. A typical electronic target response, at a distance of 3 m, illuminated by 1 W at 800 MHz from a Tx antenna with a gain of 9 dBi, and received at 1600 MHz from an Rx antenna with a gain of 10 dBi, is approximately Ptarget=−90 dBm. For 10 W transmit power and a distance of 20 m, the target response drops to Ptarget=−130 dBm. Assume that Ptrans=40 dBm at f0 and that the coupling directly between Ptrans and Prec (at all frequencies, for simplicity) is ΔPcoupled=30 dB. If the amplifier generates Ptrans=−30 dBm at 2f0 and if the filter is capable of rejecting 2f0 at the output of the amplifier by 60 dB, then the transmitter-generated harmonic that couples directly to the receiver is
                                                                                          P                  system                                ⁡                                  (                                      2                    ⁢                                          f                      0                                                        )                                            =                            ⁢                                                                    P                                          tran                      .                                                        ⁡                                      (                                          2                      ⁢                                              f                        0                                                              )                                                  -                                                                                              S                      21                      filter                                        ⁡                                          (                                              2                        ⁢                                                  f                          0                                                                    )                                                                                        -                                  Δ                  ⁢                                                                          ⁢                                                            P                      coupled                                        ⁡                                          (                                              2                        ⁢                                                  f                          0                                                                    )                                                                                                                                              =                            ⁢                                                                                          -                      30                                        ⁢                                                                                  ⁢                    dBm                                    ⁢                                                                          -                                      60                    ⁢                                                                                  ⁢                    dB                                    -                                      30                    ⁢                                                                                  ⁢                    dB                                                  =                                  120                  ⁢                                                                          ⁢                  dBm                                                                                        (        1        )            which is above the target response (Psystem>Ptarget) by 10 dB, for a distance of 20 m. This scenario is illustrated in FIG. 3, for a finite frequency band fα to fβ over which the target emits a measurable harmonic response. As depicted, the target response at 2f0 is masked by the system-generated harmonic distortion at 2f0; thus, the target cannot, in theory, be detected.
To reduce the system-generated distortion Psystem to a level below the target response Ptarget, several approaches can be taken:
The rejection provided by the lowpass filter can be increased. The tradeoff is increased passband loss. The signal will need to traverse additional filter elements that will not only increase rejection in the stopband but will also increase loss in the passband. Also, lowpass filtering is a fixed solution that does not allow tuning the circuit to reject particular harmonics (e.g. if f0 and 2f0 are not known).
A bandstop filter can be substituted for the lowpass filter. The tradeoff is a periodic pattern in frequency for the passband, which may be undesirable if the receiver is sensitive to high-frequency noise and/or higher harmonics. Like the lowpass filter, the bandstop filter is a fixed-frequency solution.
A tunable lowpass filter may be substituted for the fixed lowpass filter. This solution is still not ideal because tuning is typically accomplished mechanically, which is slow compared to the change-of-frequency required for a practical radar system such as a stepped-frequency radar, electronically, which degrades the linearity of the transmitted signal.
A filter bank may be implemented with electronic switches. However, insertion of switches to select between multiple filters will increase the loss of the overall filtering structure, and/or degrade the linearity of the transmitted signal.
Further improvements would be useful.